Deadly ricin sent to US senator

An envelope addressed to a senator has tested positive for ricin, a potentially fatal poison, congressional officials said, heightening concerns about terrorism a day after a deadly bombing killed three and left more than 170 injured at the Boston Marathon.

The office of Republican Senator Roger Wicker of Mississippi issued a statement saying "any inquiries regarding member security must be directed to the United States Capitol Police".

Capitol Police had no immediate comment.

But Majority Leader Harry Reid told reporters of the test, and other politicians said they had been briefed by the office of the Senate Sergeant At Arms.

Mail addressed to congressional offices in Washington has been routinely screened at an off-site facility since anthrax attacks in 2001, and there was no evidence it was ever delivered to Mr Wicker's office.

Milt Leitenberg, a University of Maryland bioterrorism expert, said ricin is a poison derived from the same bean that makes castor oil, and must be ingested to be fatal.

Democrat Senator Dick Durbin said the letter was discovered at a mail processing plant in Prince George's County in suburban Maryland.

"Luckily, this was discovered at the processing centre off premises," he said.

Mr Durbin said all mail to senators is "roasted, toasted, sliced and opened" before it ever gets to them.